The fluidized catalytic cracking of hydrocarbons is the main stay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. Large hydrocarbon molecules, associated with the heavy hydrocarbon feed, are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed.
The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons has been in existence since the early 1940's. The basic components of the FCC process include a reactor, a regenerator and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes entrained hydrocarbons from the catalyst by counter-current contact with steam or another stripping medium. The FCC process is carried out by contacting the starting material whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons with a catalyst made up of a finely divided or particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction. During the cracking reaction, coke will be deposited on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is traditionally transferred from the stripper to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content, relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
In order to obtain catalyst flow between the reactor and regenerator, a certain pressure balance is needed between the two vessels. Catalyst transferred to the regenerator is driven by the pressure in the upper section of the reactor vessel and the head of catalyst in the flow path from the reactor vessel to a control valve that controls the amount of catalyst entering the regenerator. Catalyst transfer from the regenerator to the riser is similarly driven by the pressure in the regenerator vessel and the head of catalyst in the flow path between the regenerator vessel and the bottom of the riser. There is usually about a 4-10 psi difference in pressure between the bottom of the riser and the top of the reactor vessel. Increases in pressure in the regenerator result in a corresponding increase in the pressure in the reactor. Any pressure increase of the regenerator relative to the reactor will be limited by the catalyst head in the flow path of the catalyst from the reactor to the regenerator. This interdependence of pressure often does not provide the most favorable pressure conditions in both the reactor and the regenerator. Low pressures are generally favored in the riser. Low pressures in the riser improve volatilization of the hydrocarbons and product yields. On the other hand, high pressures are favored in the regenerator to increase the coke burning capacity and improve the kinetics for coke burning. However, it is not possible to raise regenerator/reactor pressure differentials beyond a limited range due to the elevation of the available static catalyst head in the reactor vessel and stripper.
The rate of conversion of the feedstock within the reaction zone is controlled by regulation of the temperature of the catalyst, activity of the catalyst, quantity of the catalyst (i.e., catalyst to oil ratio) and contact time between the catalyst and feedstock. The most common method of regulating the reaction temperature is by regulating the rate of circulation of catalyst from the regeneration zone to the reaction zone which simultaneously produces a variation in the catalyst to oil ratio as the reaction temperatures change. That is, if it is desired to increase the conversion rate, an increase in the rate of flow of circulating fluid catalyst from the regenerator to the reactor is effected.
The hydrocarbon product of the FCC reaction is recovered in vapor form and transferred to product recovery facilities. These facilities normally comprise a main column for cooling the hydrocarbon vapor from the reactor and recovering a series of heavy cracked products which usually include bottom materials, cycle oil, and heavy gasoline. Lighter materials from the main column enter a concentration section for further separation into additional product streams.
Improvements in the reduction of product losses and the control of regeneration temperatures have been achieved by providing multiple stages of catalyst stripping and raising the temperature at which the catalyst particles are stripped of products and other combustible compounds. Both of these methods will increase the amount of low molecular weight products that are stripped from the catalyst and will reduce the quantity of combustible material in the regenerator. A variety of arrangements are known for providing multiple stages of stripping and heating the spent catalyst to raise the temperature of the stripping zone. With increasing frequency it is being proposed to raise the temperature of the stripping zone by mixing the spent catalyst with hot regenerated catalyst from the regeneration zone.
As the development of FCC units has advanced, temperaures within the reaction zone were gradually raised. It is now commonplace to employ temperatures of about 525.degree. C. (975.degree. F.). At higher temperatures, there is generally a loss of gasoline components as these materials crack to lighter components by both catalytic and strictly thermal mechanisms. At 1025.degree. F. (525.degree. C.), it is typical to lose 1% on the potential gasoline yield due to gasoline components thermally cracking into lighter hydrocarbon gases. As temperatures increase, to say 1025.degree. F. (550.degree. C.), most feedstocks lose up to 6% or more of the gasoline yield due to thermal cracking of gasoline components.
One improvement to FCC units, that has reduced the product loss by thermal cracking, is the use of riser cracking. In riser cracking, regenerated catalyst and starting materials enter a pipe reactor and are transported upward by the expansion of the gases that result from the vaporization of the hydrocarbons, and other fluidizing mediums if present, upon contact with the hot catalyst. Riser cracking provides good initial catalyst and oil contact and also allows the time of contact between the catalyst and oil to be more closely controlled by eliminating turbulence and backmixing that can vary the catalyst residence time. An average riser cracking zone today will have a catalyst to oil contact time of 1 to 5 seconds. A number of riser designs use a lift gas as a further means of providing a uniform catalyst flow. Lift gas is used to accelerate catalyst in a first section of the riser before introduction of the feed and thereby reduces the turbulence which can vary the contact time between the catalyst and hydrocarbons.
The benefits of using lift gas to pre-accelerate and condition regenerated catalyst in a riser type conversion zone are well known. Lift gas typically has a low concentration of heavy hydrocarbons, i.e. hydrocarbons having a molecular weight of C.sub.3 or greater are avoided. In particular, highly reactive type species such as C.sub.3 plus olefins are unsuitable for lift gas. Thus, lift gas streams comprising steam and light hydrocarbons are generally used.
Riser cracking whether with or without the use of lift gas has provided substantial benefits to the operation of the FCC unit. These can be summarized as a short contact time in the reactor riser to control the degree of cracking that takes place in the riser and improved mixing to give a more homogeneous mixture of catalyst and feed. A more complete distribution prevents different times for the contact between the catalyst and feed over the cross-section of the riser such that some of the feed contacts the catalyst for a longer time than other portions of the feed. Both the short contact time and a more uniform average contact time for all of the feed with the catalyst has allowed overcracking to be controlled or eliminated in the reactor riser.
Unfortunately, much of what can be accomplished in the reactor riser in terms of uniformity of feed contact and controlled contact time can be lost when the catalyst is separated from the hydrocarbon vapors. As the catalyst and hydrocarbons are discharged from the riser, they must be separated. In early riser cracking operations, the output from the riser was discharged into a large vessel. This vessel serves as a disengaging chamber and is still referred to as a reactor vessel, although most of the reaction takes place in the reactor riser. The reactor vessel has a large volume. Vapors that enter the reactor vessel are well mixed in the large volume and therefore have a wide residence time distribution that results in relatively long residence times for a significant portion of the product fraction. Product fractions that encounter extended residence times can undergo additional catalytic and thermal cracking to less desirable lower molecular weight products.
In an effort to further control the contact time between catalyst and feed vapors, there has been continued investigation into the use of cyclones that are directly coupled to the end of the reactor riser. This direct coupling of cyclones to the riser provides a quick separation of most of the product vapors from the catalyst. Therefore, contact time for a large portion of the feed vapors can be closely controlled. One problem with directly coupling cyclones to outlet of the reactor riser is the need for a system that can handle pressure surges from the riser. These pressure surges and the resulting transient increase in catalyst loading inside the cyclones can overload the cyclones such that an unacceptable amount of fine catalyst particles are carried over with the reactor vapor into downstream separation facilities. Therefore, a number of apparatus arrangements have been proposed for direct coupled cyclones that significantly complicate the arrangement and apparatus for the direct coupled cyclones, and either provide an arrangement where a significant amount of reactor vapor can enter the open volume of the reactor/vessel or compromise the satisfactory operation of the cyclone system by subjecting it to the possibility of temporary catalyst overloads.
Although direct coupled cyclone systems can help to control contact time between catalyst and feed vapors, they will not completely eliminate the presence of hydrocarbon vapors in the open space of a reactor vessel. Product vapors are still present in this open space from the stripped hydrocarbon vapors that are removed from the catalyst and pass upwardly into an open space above the stripping zone. The amount of hydrocarbon vapors is also increased by direct coupled cyclone arrangements that allow feed vapors to enter the open space that houses the cyclones. Since the dilute phase volume of the reactor vessel remains unchanged when direct coupled cyclones are used and less hydrocarbon vapors enter the dilute phase volume from the riser, the hydrocarbon vapors that do enter the dilute phase volume will be there for much longer periods of time when a direct coupled cyclone system is used. (The terms "dense phase" and "dilute phase" catalysts as used in this application are meant to refer to the density of the catalyst in a particular zone. The term "dilute phase" generally refers to a catalyst density of less than 20 lbs/ft.sup.2 and the term "dense phase" refers to catalyst densities above 30 lbs/ft.sup.2. Catalyst densities in the range of 20 to 30 lbs/ft.sup.2 can be considered either dense or dilute depending on the density of the catalyst in adjacent zones or regions.) In other words, when a direct coupled cyclone system is used, less product vapors may enter the open space of the reactor vessel, but these vapors will have a much longer residence time in the reactor vessel. As a result, any feed components left in the reactor vessel are substantially lost to overcracking.
The very low gas flow rate through the reactor vessel can also promote coke deposition on the interior of the vessel. The long residence time of heavy hydrocarbons at relatively high temperature in the upper section of the reactor vessel promotes the formation of coke. These coke deposits interfere with the function of the reactor vessel by forming thick deposits on the interior of the vessel thereby insulating and locally cooling portions of the metal shell. Such locally cooled portions promote the condensation of corrosive materials that can damage the reactor vessel. In addition, other problems are created by the large coke deposits which can, from time to time, break off in large chunks and block the flow of catalyst through the vessels or conduits.
One apparatus that has been known to promote quick separation between the catalyst and the vapors in the reactor vessels is known as a ballistic separation device which is also referred to as a vented riser. The structure of the vented riser in its basic form consists of a straight portion of conduit at the end of the riser and an opening that is directed upwardly into the reactor vessel with a number of cyclone inlets surrounding the outer periphery of the riser near the open end. The apparatus functions by shooting the high momentum catalyst particles past the open end of the riser where the gas collection takes place. A quick separation between the gas and the vapors occurs due to the relatively low density of the gas which can quickly change directions and turn to enter the inlets near the periphery of the riser while the heavier catalyst particles continue along a straight trajectory that is imparted by the straight section of riser conduit. The vented riser has the advantage of eliminating any dead area in the reactor vessel where coke can form while providing a quick separation between the catalyst and the vapors. However, the vented riser still has the drawback of operating within a large open volume in the reactor vessel.